1. Field of the Invention:
This invention relates in general to electrically driven centrifugal submersible pumps, and in particular to a pump system that has a pressure monitoring device and also means for isolating the pressure monitoring device from ground so as to allow testing of the insulation resistance of the power cable and motor winding.
2. Description of the Prior Art:
This invention deals with electrical submersible pump systems, particularly those used with oil wells. The pump system has a centrifugal pump with a number of stages located downhole. An electric motor will be coupled to the pump downhole. A power cable with three separate conductors extends from the surface to the pump. Three-phase alternating current (AC) will be supplied from the surface to the motor to drive the pump.
During installation, and also at other times, it is desirable to test the insulation resistance of the motor winding and the power cable. The three phases of the motor winding will be tied together into the three power conductors. High DC voltage will be applied at the surface between the power conductors and ground. If there is any leakage current to ground through the insulation of the power conductors or the motor, then the surface testing device, known as a meggar, will show a resistance below that desired. This megging test indicates the dielectric condition of the insulation system.
Testing for insulation leakage becomes a problem if the pump has a downhole pressure monitoring system of the type that is connected to ground. In this type of system, typically the downhole package will include a pressure transducer whose resistance varies with the pressure. One lead of the pressure transducer will be connected to ground. The other lead will be connected through a filter to the motor windings. At the surface, a measuring means will superimpose a DC voltage or current onto the AC power conductors. A filter at the surface blocks the measuring circuit from the AC power. This DC voltage causes a DC current to pass through the uphole and downhole filters and flow through the pressure transducer to ground. This provides a measurement of the resistance of the pressure transducer and thus an indication of the downhole pressure.
Because the pressure transducer connects to ground, the system cannot be tested for insulation leakage unless the pressure transducer is isolated from the motor windings. In U.S. Pat. No. 4,178,579, a relay is utilized downhole. This relay may be activated by a different voltage transmission than the voltage utilized for the pressure transducer. When activated, the relay disconnects the pressure and temperature monitoring system from the motor winding. This allows the system to be tested for insulation resistance. While workable, a disadvantage is that the relay may malfunction downhole, preventing the pressure and temperature system from working properly.